The present invention generally relates to a method and apparatus for reproducing an image of an original pattern onto a photosensitive material by controlling an optical system for exposure through image signals obtained from photoelectrical scanning of the original pattern, and more particularly to a method and apparatus for reproducing a halftone image of a variety of screen angles, from a continuous-toned original pattern without any use of a contact screen.
In reproducing halftone images by scanning, such as in a color scanner or a color facsimile reproducer, most of the prior techniques have utilized a contact screen in one form or another because of the simple operation of such a system. According to one of those techniques, for example, a contact screen is placed in contact with a photosensitive material, on which a halftone image is recorded throgh the contact screen by light suitably modulated with image signals.
This technique, however, poses various problems. For instance, it is very difficult to place a contact screen and a lithographic emulsion in contact with each other, particularly when a drum-type scanner is used. The uniformity of the halftone dot size cannot, therefore, be insured. Furthermore, this technique in general requires a large investment for the apparatus itself.
In an alternative technique to solve the above-described problem, the contact screen is positioned in the light path away from the lithographic film. However, it has ben pointed out that a fatal drawback of this technique is that another drum, to be used exclusively for the contact screen, has to be provided besides two other drums for wrapping a lithographic film and an original pattern. Even by this technique the problem of the uniformity of the halftone dot size still remains unsolved.
In contrast with these prior art approaches employing a contact screen, some of the techniques now in wide use are aimed at electrically forming halftone dots in such a manner that a unit area of vignet halftone dots is divided into minute parts and different density values in such divided parts are memorized in advance. These memorized density values are read out in synchronism with scanning of an original pattern, serving to form halftone images in association with obtained image signals. The resulting halftone dots, however, are usually observed to have notched contours in cases where the divided parts are too large.
In order to eliminate various disadvantages often encountered in the prior art techniques, a certain useful and improved apparatus for reproducing halftone images without using a contact screen has recently been proposed by the applicant of the present invention. As will be referred to hereinafter, this invention, disclosed in published Japanese Patent Laid-Open Specification No. 51-88301, filed Jan. 29, 1975, has also turned out to have some inadequacies in other respects in spite of its material advantages over the prior art.
For example, halftone dots are recorded aligned in only one direction, i.e., always in the same direction as a scanning line. For this reason, it has been impossible to form a halftone plate of any other screen angle. Further, the resolving power is relatively low since it is impossible to reproduce other than one row of halftone dots with one scanning operation. In other words, the ratio of the width of the scanning line to the halftone dot interval is always limited to 1 to 1, according to this prior apparatus.